Author Topic: 10x14 post and pier foundation question  (Read 1990 times)

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Offline cbc58

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10x14 post and pier foundation question
« on: August 03, 2017, 07:27:00 AM »
On the 10x14 post and pier foundation - what size lumber are the beams made of?  2x8 or 2x10?  are the doubled up or single? 

http://www.countryplans.com/jshow.com//y2k/listings/7.html

Also - are the metal brackets secured into the precast pier blocks or simply resting on top?  The blocks that I am looking at don't have a hole - just a place for a 2X to sit in...   

thinking of building a shed with this system as I have no power or water at our land...  tks

 
« Last Edit: August 03, 2017, 07:46:00 AM by cbc58 »

Offline akwoodchuck

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Re: 10x14 post and pier foundation question
« Reply #1 on: August 03, 2017, 05:31:34 PM »
I like the blocks with the adjustable saddles for small sheds....usually just drop a 4x6 or 4x8 beam right in and go....easy peasy to re- level as necessary.

Offline cbc58

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Re: 10x14 post and pier foundation question
« Reply #2 on: August 04, 2017, 10:04:27 AM »
i can't find those blocks around here (at the big box stores).  they don't sell 4x6x14 beams either - 12' is the longest they have.  they said they could get a 16' but it's just me doing the work and that's too heavy to move by myself since it has to be carried uphill into the woods.  think my best bet is 2x8 or 2x10's sistered together with plywood between... 

Offline ChugiakTinkerer

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Re: 10x14 post and pier foundation question
« Reply #3 on: August 09, 2017, 08:38:04 AM »
You can probably find the 4x6 you need at a lumber yard.  Even if you have to buy a 16' one, I'd recommend staying with pressure treated sawn wood.  Cut it to size before hauling it up the hill.  You'll likely find that the lumber yard has better treatment too, which will ensure that your beams hold up to weather and rot much longer.
My cabin build thread: Alaskan remote 16x28 1.5 story

Offline Don_P

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Re: 10x14 post and pier foundation question
« Reply #4 on: August 10, 2017, 01:47:05 AM »
A built up beam is stronger than a solid sawn one... distribution of defects. Plywood adds effectively nothing to the strength of  built up beam. Depth adds strength and stiffness fast, width is an expensive and often ineffective way to make a beam stronger. If the design allows "deeper is cheaper". In the engineering calcs, width is simply multiplied, depth is squared, this builds strength fastest. On the other side of those equations span is squared, span kills a beam fast.

Offline ChugiakTinkerer

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Re: 10x14 post and pier foundation question
« Reply #5 on: August 10, 2017, 09:13:36 AM »
All good points Don.  For the application where one is using a 3.5" bracket, like the concrete blocks with the bracket cast in, it seems a wash between a 4x and a built-up beam of 2 2x.  As you said, the plywood offers no strength, so you're looking at 3.5" of a single board versus 3" of a built-up beam.  The 15% increase in strength you get from the built-up is about matched by the 16% increase in width you get from the 4x.  In that specific application I think it would be better to have PT, even in just one sawn board, than to have a plywood spacer in the built-up beam that would much more be susceptible to moisture damage.
My cabin build thread: Alaskan remote 16x28 1.5 story

Offline cbc58

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Re: 10x14 post and pier foundation question
« Reply #6 on: August 18, 2017, 07:31:11 AM »
thanks for the info.  have another question: how far can you cantilever the floor decking over the support beams?  is 1 foot to  1.5 feet acceptable?  based on the particulars of my site, the beams look like they are going to be 9'6" apart and wondering how wide of a structure is wise to build based with the overhang on each side of the beams...

Offline akwoodchuck

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Re: 10x14 post and pier foundation question
« Reply #7 on: August 18, 2017, 12:19:52 PM »
Rule of thumb for load-bearing cantilevers is you can go out the depth of the joist....so if you want to go past the beam a foot, you should use 12" joists.....

Offline cbc58

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Re: 10x14 post and pier foundation question
« Reply #8 on: August 18, 2017, 01:26:08 PM »
good to know.  tks

Offline ChugiakTinkerer

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Re: 10x14 post and pier foundation question
« Reply #9 on: August 18, 2017, 02:11:37 PM »
Rule of thumb for load-bearing cantilevers is you can go out the depth of the joist....so if you want to go past the beam a foot, you should use 12" joists.....
Sorry to go off topic but your comment just gave me an AHA! moment.  I'm aware of the cantilever rule of thumb, but it wasn't until I read your comment and the words load-bearing jumped out at me, but that's the same rule of thumb when you're supporting a load bearing wall or column.  It didn't click until now that there's only one rule of thumb here, not two.
My cabin build thread: Alaskan remote 16x28 1.5 story

Offline Don_P

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Re: 10x14 post and pier foundation question
« Reply #10 on: August 18, 2017, 04:05:43 PM »
Go one deeper, with most materials, most bracing, etc make the load path no shallower than 45 degrees.
There is a cantilever chart, table R502.3.3(1)
https://codes.iccsafe.org/public/document/code/594/10054242

Offline cbc58

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Re: 10x14 post and pier foundation question
« Reply #11 on: August 26, 2017, 03:33:23 AM »
Looking at various ways to attach 2x8's to a 6x6 which will be the support beam for 11x14 building.  I know one of the preferred methods is to notch out the 6x6's and double up the 2x8's and carriage bolt them in. 

How about using just 1/2" x 10" carriage bolts and putting a 2x8 on each side of the 6x6?  2-3 bolts per post?  Is that acceptable structurally?  The weight load of people and items inside the structure (not including the structure) is estimated to be max around 1400 lbs.  That would be on a rare occasion and it would likely be around 500 lbs most times.

6 support posts total - 3 for each support beam.   Would this work from a structural standpoint?  Tks in advance.
 




Offline MountainDon

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Re: 10x14 post and pier foundation question
« Reply #12 on: August 26, 2017, 04:54:00 AM »
Couple of thoughts on that... A 1/2" carriage bolt is threaded full length, so ends up being about equal to a 3/8" dia. bolt shank. A 1/2" machine bolt and nut with washers provides a true 1/2" diameter full length. That might be important with the weight acting in shear on the bolt.

Bolting the 2x on the sides like that also means one of the strength limitations is the 2x itself. If a split develops due to a flaw or whatever, the split may pass through a drilled hole and in effect reduces the 2x8 to maybe a 2x3. With the post notched you provide a shelf for the weight to sit on and the bolt acts only in tension to hold the 2x and post clamped together.

That said I am not an engineer and it is a small structure......  ???
Just because something has been done and has not failed, doesn’t mean it is good design.

Offline cbc58

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Re: 10x14 post and pier foundation question
« Reply #13 on: August 26, 2017, 09:51:24 AM »
tks Mtn. Don.  found this link on difference between carriage and machine bolts.  http://www.finehomebuilding.com/2008/01/17/is-a-carriage-bolt-as-strong-as-a-machine-bolt

Didn't even consider tensile strength in my application and was concerned about shear strength.

Offline Don_P

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Re: 10x14 post and pier foundation question
« Reply #14 on: August 27, 2017, 04:36:57 AM »
Bolting the girder on the sides is prohibited for a deck for the reasons MD mentioned, the bolt induces splitting. Also bolts need to be no more than 5" apart across the grain to avoid splitting during drying, a concern with wider members. I put the lower bolt 2" up off the bottom edge to stay within min edge distance requirements while trying to keep from rising off the notch.

Offline arcedvillage

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Re: 10x14 post and pier foundation question
« Reply #15 on: August 30, 2017, 05:03:13 AM »
Hello everyone,
I am 100% new here, but I have read the other (older) thread on this topic, and found little in the way of complete answer. The links lead nowhere (we ARE talking about a 3 year old thread, so it makes sense).

So, the plan for our post piers is to go deep enough for small amounts of geothermal, the building will be 5-10' off the ground (in an effort to preserve the local marsh and forest ecosystem), and the pier posts will be concrete with rebar reinforcement skeleton. In the old thread it was said that the frost line is a starting point for depth, not the goal. I find this 100% logical and thank the posters for that advice! :) What I need to know now is how deep should I go?

I am not necessarily asking if anyone here KNOWS that answer, but if anyone here knows where I can LOOK for that formula. I am a self taught engineer with a Masters in Rural Development, so once I know the formula the math will be a snap. I thank all contributes to this and welcome anyone who wishes to poke holes / smack me upside my fool head. :P I am admittedly a newbie in this particular field, and defer to your expertise.

Offline ChugiakTinkerer

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Re: 10x14 post and pier foundation question
« Reply #16 on: August 30, 2017, 08:23:40 AM »
Hello arcedvillage, welcome to the forum!  I'm not claiming any expertise, so take what I say with a grain of salt.  You want the anchor portion of your column to be below the frost line, and with a big enough footprint so that it supports the entire weight of the column.  How big that is depends on what the soil or ground material is that it's sitting in.  Going deeper doesn't gain you any load bearing capability unless there's better material below.

Suppose you have 5' of unconsolidated sediments resting atop bedrock, and a frost depth of 3'.  You could gain a lot of load-bearing capability by going all the way to bedrock.  But if those sediments are clean sand and gravel, there wouldn't be much gain as they are already very capable material.  How deep you go from a load-bearing standpoint is determined entirely by the depth profile of your ground conditions.

Going deeper for a pier column does have the benefit of making the column more resistant to lateral and uplift forces.  In areas prone to earthquakes or high winds that might be essential to keeping the structure in place.  Or perhaps in an area prone to flooding, a deeper pier footing would increase the likelihood of a structure staying in place.  In any of those cases you would be well advised to consult an engineer.  Depending on what you have planned that may be required anyhow, as in most jurisdictions that require building permits you have to build to current code.  That means either an approved continuous foundation design or plans that have been stamped by an engineer licensed in your state.

There are alternative methods to a traditional concrete pier, but further discussion on those are best suited for another thread.  Why don't you start a new topic, either in this forum or the Owner-Builder one, and describe your project and where it's located.

My cabin build thread: Alaskan remote 16x28 1.5 story

Offline MountainDon

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Re: 10x14 post and pier foundation question
« Reply #17 on: August 30, 2017, 11:37:03 AM »
..... frost line is a starting point for depth, not the goal. I find this 100% logical and thank the posters for that advice! :) What I need to know now is how deep should I go?

There is normally no reason to go deeper than frost depth. The reason we go to frost depth is to prevent the earth from freezing below the footing and the resultant potential for the footing to be pushed upwards. One reason to go deeper would be to construct a basement.

However, there are foundations known as pilings, not piers, that do go deeper. They are most often driven into the earth with a pile driver. Some piles may be driven to bedrock. Other piling are driven deep enough to where the friction between the skin of the pile and the ground is what supports the structure. These are often 12 times, and more,  longer than the width of the piling. Some pilings are cast in augured holes. How deep they are placed depends upon the soil composition.

If you have seen photos of some of the homes along coasts that appear to be raised on piers, you are most likely seeing pilings.

Because of their depth, pilings will also have much greater lateral resistance to movement. Piers that are installed to frost depth will have less lateral resistance than a deeper pile, depending on soil and other factors.


....the building will be 5-10' off the ground

That height takes you right smack dab into the realm of engineering. How to deal with the variables is what engineers go to school for. I have not seen any DIY resources that would safely lead us DIY'ers through a step by step DIY engineering process. If you need to, or want to, build elevated like that I believe you are best to pay for a licensed engineers design.
Just because something has been done and has not failed, doesn’t mean it is good design.

 

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